The present invention relates to an image forming apparatus such as a copying machine, a printer and a facsimile machine using an electrophotographic method. In particular, the present invention relates to an image forming apparatus of an intermediate transfer method with which a visible image formed on an image bearing member is first transferred to an intermediate transfer member, and then transferred to a transfer material to obtain image formation.
An image forming apparatus using an intermediate transfer member is an apparatus for obtaining a full color image formation (a copy or a print). This is attained by repeating a process of primarily transferring a toner image (that is, a visible image) formed on a photosensitive member (an image bearing member for toner images of a plurality of colors), and then secondarily transferring the primarily transferred toner images collectively to a recording material such as paper (a transfer material).
A known image forming apparatus of this type is the one employing an intermediate transfer belt. This image forming apparatus is provided with a plurality of image forming portions (stations) of yellow (Y), magenta (M), cyan (C) and black (K) along the intermediate transfer belt. Each image forming portion has a photosensitive drum (a drum-shaped electrophotographic photosensitive member) as an image bearing member.
An image is formed by uniformly charging a surface of a photosensitive drum by a primary charger, forming an electrostatic latent image on the photosensitive drum by exposing an image pattern using an exposure device, and developing the latent image by a developing device to visualize it as a toner image.
Toner images of four colors formed on a plurality of photosensitive drums in this way are primarily transferred onto the intermediate transfer belt while being superimposed over it in a primary transfer portion, in which the photosensitive drums and the intermediate transfer belt contact, by supplying transfer charge from primary transfer means contacting the intermediate transfer belt. Then, a secondary transfer portion is formed by contacting secondary transfer means, which was separated from the intermediate transfer belt, with the intermediate transfer belt, and transfer charge is supplied by the secondary transfer means. The toner images of four colors on the intermediate transfer belt are thereby secondarily transferred collectively onto a recording material such as paper supplied to a secondary transfer portion.
Successively, the recording material on which the toner images of four colors have been transferred is conveyed to a fixing device. A full-color image is formed on the recording material by mixing the toner images of four colors and simultaneously fixing them on the recording material while they are passing through the fixing device.
Toner remains on the intermediate transfer belt by the secondary transfer. A cleaning bias impressing roller, which was separated from the intermediate transfer belt during the image formation on the intermediate transfer belt, contacts the intermediate transfer belt, and is charged in a positive polarity opposite to a normal charging polarity of the toner. This secondary transfer residual toner is thereby transferred to the photosensitive drum in the primary transfer portion when it reaches the primary transfer portion by the movement of the intermediate transfer belt. Then, the secondary transfer residual toner is collected in the same manner as primary transfer residual toner by cleaning means of the photosensitive drum.
Incidentally, in recent years, in order to miniaturize an image forming apparatus and save energy consumed by the same, it has been attempted to devise an image forming apparatus without a cleaner for the photosensitive drum by employing a system of cleaning simultaneous with developing. However, the above mentioned conventional image forming apparatus has the following problems.
If there is no drum cleaner, it is likely that a primary charger will be stained because transfer residual toner or re-transfer toner after passing a primary transfer nip portion may become stuck to the primary charger via the photosensitive drum. In order to prevent such a stain, it is necessary to transfer the transfer residual toner or the like from the photosensitive drum onto the intermediate transfer belt in the primary transfer portion and collect it using cleaning means or the like of the intermediate transfer belt.
However, there is a case in which, when transfer charge is being supplied to the primary transfer means provided in each station, for example, an emergency stop operation for image formation may be taken only in the yellow station, the transfer charge continues to be supplied in the primary transfer portion in the magenta station, the cyan station, and even the primary transfer portion of the black station that follows the yellow station as subsequent image formation processes, as in the yellow station. This becomes a factor for reducing an operating life of the photosensitive drum. In addition, this re-transfers a toner image formed in the yellow station to a photosensitive member in the downstream side, which possibly becomes a factor for mixed colors.
FIG. 7 shows a relationship between a transfer residual rate and a re-transfer rate with respect to a transfer electric current value. Here, the transfer residual rate is a ratio of a transfer residual toner density on the photosensitive drum to a sum of a toner density on the intermediate transfer member and a transfer residual toner density on the photosensitive drum after a solid image formed on the photosensitive drum is primarily transferred onto the intermediate transfer member. In addition, the re-transfer rate is a ratio of a re-transfer toner density on the photosensitive drum of a next primary transfer portion to a sum of a toner density on the intermediate transfer member and a re-transfer toner density on the photosensitive drum of the next primary transfer portion after a solid image transferred to the intermediate transfer member in the primary transfer portion passes the next primary transfer portion. Both of these toner densities are measured by a densitometer (type number 404) manufactured by X-rite Incorporated.
In the case of an image forming apparatus that is not provided with a cleaner in a photosensitive drum, the amount of toner on the photosensitive drum after an image passes a primary transfer portion of each station must be as small as possible. However, there is an optimum primary transfer electric current value (e.g., 22 xcexcA in FIG. 7) for minimizing a transfer residual rate, and transfer electric current must be made small in order to minimize a re-transfer rate. Thus, it is difficult to minimize both the transfer residual rate and the re-transfer rate at a single transfer electric current value.
Therefore, it is an object of the present invention to provide an image forming apparatus that, in forming an image on a transfer material via a plurality of image bearing members and an intermediate transfer member, even if an emergency stop operation for image formation is taken due to a failure of transfer material conveyance, an image formation error or the like, prevents a stain of charging means of an image bearing member or that of secondary transfer means due to a toner image existing on the image bearing member, and further is capable of realizing the extension of an operating life of the image bearing member and the prevention of mixed colors between one image forming portion and the other image forming portion.
In order to attain the above-mentioned object, an image forming apparatus comprises:
a plurality of image bearing members;
a plurality of image forming means for forming an image on the plurality of image bearing members;
a plurality of primary transfer means for transferring the images on the plurality of image bearing means onto an intermediate transfer member;
secondary transfer means for transferring the image on the intermediate transfer member onto a transfer material; and
control means for controlling the primary transfer means to operate until an image area on the image bearing member on which an image is being formed at the point when an image formation stop instruction is issued passes at least a transfer region in the primary transfer means opposing to the image bearing member.
In addition, an image forming apparatus in accordance with another aspect of the present invention comprises:
a plurality of image bearing members;
a plurality of image forming means for forming images on the plurality of image bearing members;
a plurality of primary transfer means for transferring the images on the plurality of image bearing members onto an intermediate transfer member;
secondary transfer means for transferring the images on the intermediate transfer member onto a transfer material; and
control means for controlling the plurality of primary transfer means to operate until an image area on an image bearing member on which an image is being formed at the point when an image formation stop instruction is issued at least passes a transfer region in primary transfer means opposing to the image bearing member, and controlling the secondary transfer means not to operate or controlling the secondary transfer means so that an electric field opposite to that which was generated at the time of transfer operation is generated when the image area transferred onto the intermediate transfer member by the operation of the primary transfer means passes at least a transfer region in the secondary transfer means.